The thermal ink transfer printing system is now being widely employed and improved as a quiet printing system for printers of office machines and other various fields. The thermal ink transfer printing system, especially the sublimation type thermal ink transfer system, in superior to the melting ink transfer system in that tone gradation can be easily and accurately reproduced, and therefore development of this system is proceeding for application in the image copying systems for video printers and the like. With a sublimation type ink, however, reproduction of tone gradation is difficult when the binder of the sublimation type ink layer has excessively high affinity to the substrate film, since the ink cannot be easily sublimed. When a binder which is suitable to sublimation type ink is selected, sometimes the adhesion of the ink layer to the substrate film is impaired and the whole ink layer is transferred to the image receiving sheet. In order to improve adhesion of the ink layer and the base film, and achieve an accurate transferred image (faithful reproducibility of the original), usually a primer layer is provided. However, it is essential to select a good primer layer, which is thin and uniform in thickness and has good adhesiveness to both the polyester base film and the ink layer.
In forming a primer layer on a polyester film, there is a problem that the polyester film is very thin, usually 3-7.mu., and therefore it is difficult to handle it and to apply a primer with high accurately. Accordingly, the yield of production of such printing films is very low.
We noted the fact that the applied primer layer can be made thinner and more uniform if the primer material is applied onto the polyester base film and thereafter the coated base film is stretched, and found that a good printing film can be produced by employing the above-mentioned post-application stretching and selecting a suitable primer material which has good adhesiveness to the binder for the ink layer and the primer material. Thus we accomplished the present invention.